1. Field of the Invention
The present invention relates to a fixing apparatus used with an image forming apparatus such as a copying machine, a printer, a facsimile machine and the like.
2. Related Background Art
An example of a fixing apparatus for thermally fixing a non-fixed image to a recording material is shown in FIG. 12. As shown in FIG. 12, such a fixing apparatus comprises a heat roller (heating means) 10 including therein a heater 12 acting as a heat source, and a pressure roller (pressurizing means) 11 having a rubber surface and urged against the heat roller and serves to fuse toner on a paper sheet (recording material) 6 with heat of the surface of the heat roller 10 to fix the toner to the paper sheet 6 while pinching and conveying the paper sheet by rotation of the rollers. Accordingly, in order to perform the fixing effectively, it is necessary to maintain a surface temperature of the heat roller 10 to a predetermined temperature.
In this fixing apparatus, the surface temperature of the heat roller 10 is detected by a thermistor 13 contacted with the heat roller 10, and the detected surface temperature is compared with a control temperature previously set in a temperature control device (control means) 14. If the surface temperature is lower than the control temperature, the communication to the heater 12 is started; whereas, if the surface temperature is higher than the control temperature, the communication to the heater 12 is stopped. In this way, the surface temperature of the heat roller 10 is maintained to the predetermined temperature.
Further, in order to start an image forming operation as soon as a command signal for the image formation is inputted, the fixing apparatus is brought to a stand-by condition after the power source of the apparatus is turned ON. In the stand-by condition, the surface temperature of the heat roller 10 is maintained to a predetermined stand-by temperature.
Recently, in such a fixing apparatus, in order to reduce a warming-up time period for bringing the fixing apparatus to the stand-by condition after the power source is turned ON, a thin-walled roller having small heat capacity has been used as the heat roller, thereby shortening the warming-up time period. However, even when the heat roller 10 is heated up to the predetermined temperature, if the sufficient heat is not transmitted to the pressure roller 11 urged against the heat roller 10, the good fixing ability cannot be achieved.
To avoid this, conventionally, when the fixing apparatus is warmed up, during the warming-up, immediately after the heat roller 10 reaches a certain temperature, the heat roller 10 and the pressure roller 11 are rotated to also supply the heat to the pressure roller 11 (pre-multi rotation). In this pre-multi rotation, since the heat on the heat roller 10 is absorbed to the pressure roller 11 to temporarily decrease the temperature of the heat roller 10, the timing for starting the pre-multi rotation affects an influence upon a time period during which the heat roller 10 reaches the predetermined stand-by temperature (i.e. warming-up time period).
On the other hand, since the warming-up time period governs a heat amount accumulated in the pressure roller 11 and affects an influence upon the fixing ability, conventionally, the timing for starting the pre-multi rotation has been determined on the basis of a room temperature detected by a room temperature sensor and the like. That is to say, if the room temperature is low, since the temperature of the paper sheet 6 and the temperature of the pressure roller 11 are also low, even when the temperature of the heat roller 10 is constant, the fixing ability is inferior to the fixing ability obtained when the room temperature is high. Thus, the heat amount must be accumulated in the pressure roller 11 accordingly. Therefore, the timing for starting the pre-multi rotation is delayed until the heat roller 10 reaches the predetermined high temperature, thereby increasing the warming-up time period.
On the other hand, if the room temperature is high, since there is no need to accumulate a large heat amount in the pressure roller 11, the pre-multi rotation is started before the heat roller 10 reaches the predetermined high temperature, thereby decreasing the warming-up time period.
However, since the fixing ability of the image outputted from the fixing apparatus is influenced upon the combination of various factors such as the room temperature, input voltage, heat capacities of the heat and pressure rollers, detection temperature of the thermistor, output of the heater and the like, in the above-mentioned conventional fixing apparatus, the adequate fixing ability cannot be often achieved. For example, in accordance with the conventional warming-up time period, when the room temperature is high, if the input voltage is low, the fixing ability will be worsened.
Further, if the heat capacity of the roller is greater than the design reference value, during the same warming-up time period, the increase in temperature of the roller is delayed due to the difference in heat capacity, with the result that the fixing ability is worsened.